Plant growth is controlled to a large extent by water availability from the surrounding soil to the plant. Plant available water in the surrounding soil depends on the pore sizes and the surfaces offered by the mineral component of the soil. The two components generate soil matrix suction forces, also called matrix potential.
Because plant root cells have a semi-permeable membrane at their periphery, solutes can accumulate outside of the root and generate a second potential, the osmotic potential, which affects the plant growth. Solute content of the soil, or soil salinity, is linked to the electrical conductivity of the soil. For irrigated lands in which fertilizers are added (fertigation), like greenhouse and nursery production, for soils with high salt content or for irrigated lands using a large amount of fertilizers, salts built up can have detrimental effects on plant growth and therefore, electrical conductivity is regularly monitored. If needed, excess solution is applied to leach excess salts to the environment, therefore creating environmental detrimental effects. Because of the difficulty of measuring the soil solution electrical conductivity, leaching is often carrying out in excess, therefore increasing the risk of groundwater pollution by excess fertilizers or pesticides.
Therefore, numerous efforts have been carried out to measure electrical conductivity in the soil solution to help monitoring irrigation and leaching, and to optimize plant growth. However, the technology proposed so far is based on a Wenner arrangement, i.e. an arrangement of four electrodes, inserted directly into the soil for electrical conductivity measurement. Such a procedure measures the apparent electrical conductivity of the soil solution. The apparent electrical conductivity measurement is also dependent on the soil water content, soil structure and solid phase electrical properties of the soil.
U.S. Pat. No. 3,508,148 to Enfield discloses a sensor for measuring the electrical conductivity in a soil. The sensor uses a porous glass plate with pores having a diameter in the range of ten to one hundred angstrom units. A plate electrode is placed on one side of the porous glass plate and a grid electrode is placed on the opposite side. The openings of the grid provide the exposure of the porous glass to the soil wherein it is inserted, in order to allow the porous glass plate to draw the water solution of the soil in its pores by capillarity. The electrical conductivity of the porous glass plate is measured using the electrodes. As the salinity of the soil varies, the solution drawn in the pores of the porous glass plate stabilizes to the salinity of the soil by diffusion. Consequently, the time required to reach a steady state condition in the porous glass is about eight hours which is quite long for plant growth applications, since soil may be irrigated few times a day.
Furthermore, water in a soil, or a growing medium, comprises drainage water, plant available water and plant non available water. Plant non available water corresponds to bound water and available water corresponds to water that is contained in the soil and that can be drawn by plants or growing crops. Drainage water is the excess in water that drains out of the growing medium. The salinity to be monitored in plant growth applications corresponds to the salinity of the plant available water, i.e. the growing soil's solution.
In Enfield's sensor, because of the selected pore size, the porous glass does not discriminate between plant available water, plant unavailable water and both available and unavailable water are sampled by the porous medium.